1. Field of the Invention
The present invention relates to a capacitive-discharge ignition system for internal-combustion engines, comprising an improved construction of a magneto generator, in combination with a control voltage circuit for feeding and triggering the discharge of the ignition capacitor, by means of which it is possible to optimize the capacitor charging voltage, and a single ignition trigger pulse for each revolution of the magneto generator, avoiding the use of any additional trigger coil inside or outside the same generator.
2. Description of the Related Art
The capacitive-discharge ignition systems for motor cycles and the like are formed mainly by a multipolar electric generator, normally of magneto type, which in addition to supplying the energy necessary for the low-voltage loads of the motor-cycle, via a suitable additional winding, also provides the energy for charging the ignition capacitor. A pick-up or a timing sensor coil is normally provided inside or outside the magneto generator in order to generate a control signal for triggering the ignition circuit at each revolution of the engine.
The most common capacitive-discharge ignition circuits usually comprises a winding consisting of one or more serially-connected stator coils for supplying the necessary voltage for charging an ignition capacitor. Normally the coils consist of a very high number of turns of thin wire, for example from three to four thousand turns of a wire of 0.1 to 0.15 mm diameter, necessary for bringing the ignition capacitor to a charging voltage ranging between 100 and 300 Volts. During revolution of the rotor on which permanent magnets are provided, an alternating voltage is induced in the stator coils, the positive going half-cycle outputs of which directly biases some diodes connected in series to the capacitor circuit, charging the latter to the desired voltage. When a pulse in timing relation with the engine cycle is generated by an additional coil, an electronic switch arranged in the ignition-capacitor discharge circuit is activated; the energy at the voltage to which the ignition capacitor was previously charged is then discharged onto the primary winding of the ignition coil, generating on the secondary winding a high voltage which causes an electric spark in the spark-plug of the ignition circuit. The solution described above, although most commonly used, has, however, two basic drawbacks:
a non-optimum trend of the ignition-capacitor charging voltage, which tends to have a bell-shaped curve, having a high maximum value substantially influenced by the rotational speed of the engine; PA1 the presence of a separate coil for generation of the trigger signal, inside or outside the generator, for the ignition command. The use of a separate coil for generation of the ignition trigger signal is inconvenient in many applications since any appropriate positioning of the pick-up or timing coil is very difficult due to the small overall dimensions which are generally required for these types of ignition systems.
The first drawback can be overcome by using, for example, a capacitor charging system which employs a voltage booster as described in a previous patent application, IT-MI92A002809, in the name of the same Applicant.
Solutions also exist for the second drawback as well, although they are not considered to be optimal. The most widely adopted solution is undoubtedly that shown in FIG. 1 of the accompanying drawings, which envisages the use of capacitive-discharge system combined with a generator 10 comprising a stator winding L1 and a four-pole rotor 11, as schematically shown.
As can be seen from the same FIG. 1, this solution does not require the use of any additional winding for generation of the timing signal, but it is the same ignition feed winding L1 which, in addition to charging the capacitor C by means of the diode D1 via the positive half-waves generated by L1, together with the negative half-waves, triggers an electronic switch SCR in the discharge circuit of the capacitor C comprising the primary winding Lp of the high-voltage coil, the secondary winding Ls of which is connected to the spark-plug CD. A diode D4 is connected in parallel to the primary winding Lp of the high-voltage coil, while a diode D2 and biasing resistor R1 are connected in parallel between the control electrode of the SCR switch and the outlet side of the winding L1 via a biased diode D3.
During the positive half-waves, D1, D2 and D4 are forward biased and hence the capacitor C is able to be charged while the SCR is inhibited and no current is flowing in R1, since D3 being inversely biased.
When the voltage generated by L1 becomes negative, then D3 is forward biased and, via the biasing resistor R1, the control electrode of the SCR is biased directly, being triggered, this in turn permitting discharging of the capacitor C onto the primary winding Lp and onto the high-voltage coil generating the spark in the spark-plug CD.
This solution is applicable, however, only in normal four-pole generators where two sparks phase-displaced by 180.degree. with respect to each other are generated for each revolution. Since this ignition is generally used on single-cylinder engines, which require an ignition spark for each cycle, it occurs that one of the two generated sparks, although being uninfluential on operation of the engine, nevertheless contributes to an undue power consumption and an increase of the temperature of the ignition circuit.
There exist other solutions which use multipolar magneto generators without a separate trigger coil which, combined with the circuit in FIG. 3, are able to generate a single spark per revolution, as for example shown and described in U.S. Pat. No. 4,636,671.
According to this solution, the rotor has a certain number of radially magnetized magnets circumferentially arranged adjacent to one another and having a pole of the same North or South polarity facing towards the stator coils, while remaining magnets have the usual alternation of their North and South polarities.
The stator of this generator has, moreover, a large pole shoe for the capacitor charging coil, which is of greater dimensions than the pole shoes of the remaining poles thus involving additional operations for the stator and separate winding of the coils.
As can be better seen from FIG. 5 of U.S. Pat. No. 4,636,671, the pole shoe of the capacitor charging coil has a circumferential extension greater than the single magnets, so that it short-circuits the magnetic flux between adjacent magnets when the latter have opposite polarities.
Therefore, according to this prior art solution, only in the case of adjacent poles having the same polarity is the flux able to pass through the magnetic pole shoe of the capacitor charging coil and hence the latter generate a voltage signal as shown in the same FIG. 6, this signal being compatible with the electronic ignition circuit shown in FIG. 1, thus generating a single spark per revolution of the engine.
The drawback of this solution, mainly resides in that the charging of the ignition capacitor cannot be optimized because during a 360.degree. revolution, a single positive half-wave (30.degree. in the case of a 12-poles generator) is disposable for charging the capacitor and for ignition purpose.
In addition to this drawback, by having a stator pole shoe of larger dimension, which is so different from the others, involves further problems as regards execution or the winding of the coils.
U.S. Pat. No. 4,537,174 also describes a 12-poles magneto generator in which the capacitor charging coils are wound onto two adjacent pole shoes of identical angular width and in which a cup-shaped rotor comprises a ring shaped main magnet magnetized to provide a plurality of alternate North and South poles which are arranged in succession along the entire inner surface of the circular side wall of the rotor; in this generator use is made also of a pick-up coil to generate the timing signal for triggering the ignition capacitor, which pick-up coil is positioned outside the generator, with the consequent disadvantages and drawbacks referred to above.
At present U.S. Pat. No. 4,636,671 represents the closest prior art on which the innovative features of the invention can be defined.